Carbohydrates & Lipids Fundamentals

Starch: Structure, Function & Metabolism

• Coiled Structure of Starch (Polysaccharide P)

  • Compact, helix-like architecture → packs a large amount of glucose units into a small volume
  • Significance: makes starch an ideal intracellular storage molecule (fits in plastids / amyloplasts without exerting osmotic pressure)

• Main Function: Energy Reservoir

  • Starch = primary carbohydrate store in plants → substrate for ATP production in heterotrophs
  • Transformation to a respiratory substrate
  • \alpha-amylase hydrolyses starch → soluble \alpha-glucose monomers
  • \alpha-glucose enters glycolysis → link reaction → Krebs cycle → oxidative phosphorylation → ATP
  • Overall: hydrolysis (catabolism) converts an osmotically inert polymer into metabolically active monomeric fuel

• Formation & Breakdown of Maltose (Disaccharide)

  • Condensation synthesis
  • 2 \alpha-glucose molecules align so that \text{C}1 –\text{OH} of one and \text{C}4 –\text{OH} of the other react
  • Forms an \alpha(1\rightarrow4) glycosidic bond
  • Releases one molecule of water (dehydration)
  • Hydrolytic cleavage
  • Enzyme: maltase (reversible catalyst)
  • Addition of \text{H}_2\text{O} breaks the \alpha(1\rightarrow4) bond → regenerates two \alpha-glucose molecules
  • Reaction is reversible under physiological conditions

Lipids: Overview & Classification

• Definition & General Properties

  • Broad group of non-polymeric organic compounds rich in \text{C} and \text{H}, poor in \text{O}
  • Insoluble in water (hydrophobic); soluble in organic solvents (ether, acetone, chloroform, hot alcohol)
  • Empirical representation: CnH{2}O_{2} (Oxygen proportion lower than carbohydrates)
  • Store large energy per mass owing to high C–H bond density (≈ 38\,\text{J g}^{-1})

• Three Major Classes

  1. Triglycerides (fats & oils)
  2. Phospholipids (e.g.
     lecithin)
  3. Steroids (e.g.
     cholesterol, testosterone)

Triglycerides (Triacylglycerols)

• Constituent Molecules

  • Glycerol: 3-carbon alcohol, each carbon bears one hydroxyl (–OH)
  • Fatty acid: long unbranched hydrocarbon (usually 16–18 C) terminating in carboxyl (–COOH)

• Synthesis (Condensation / Esterification)

  • Three fatty acids + one glycerol → triglyceride + 3\,\text{H}_2\text{O}
  • Bond type: ester linkage (\text{COO}–) between hydroxyl of glycerol and carboxyl of fatty acid

• Hydrolysis

  • Reverse reaction (lipase-catalysed) produces glycerol + 3 fatty acids; water consumed

• Saturated vs Unsaturated Fatty Acids / Fats

  • Saturated
  • Hydrocarbon chain fully saturated with H
  • No C=C double bonds → straight chains pack tightly → solid at 25\,^{\circ}\text{C}
  • Chemically inert; increase LDL; typical of animal fats (lard, butter)
  • Unsaturated
  • ≥1 C=C double bond → kinks prevent tight packing → liquids at 25\,^{\circ}\text{C}
  • More chemically reactive (prone to oxidation/rancidity); decrease LDL; common in plant oils (olive, sunflower)
  • Comparative table
  • Both are triglycerides; form via condensation; hydrolyse to same monomers; both store energy

Phospholipids

• Composition

  • Glycerol backbone + 2 fatty acids (hydrophobic tails) + 1 phosphate-containing group (hydrophilic head)
  • Amphipathic nature → spontaneous formation of bilayers in aqueous milieu

• Biological Significance

  • Principle structural component of cellular membranes; confers fluidity & selective permeability
  • Allow diffusion of lipid-soluble/non-polar substances; barrier to polar solutes

Steroids

• Core Structure

  • Rigid framework of four fused rings (three 6-C + one 5-C) = 17 carbons total in skeleton
  • Functional diversity from side-chain variations

• Representative Molecules

  • Cholesterol (membrane component/precursor)
  • Testosterone, progesterone, oestrogen (sex hormones)

Cholesterol

• Molecular Arrangement

  • 27 carbon atoms in fused-ring skeleton + hydrocarbon tail
  • Single hydroxyl (–OH) = hydrophilic head; rest of molecule hydrophobic
  • Amphipathic: aligns in membranes with OH near phospholipid heads, rings & tail among fatty acyl chains

• Properties

  • Insoluble in water; soluble in organic solvents
  • Adds rigidity yet maintains fluidity in phospholipid bilayers (temperature buffer)

• Metabolic & Physiological Roles

  • Precursor for
  • Bile salts → emulsification of dietary fats
  • Vitamin D → calcium homeostasis, bone/teeth integrity
  • Steroid hormones (testosterone, progesterone, cortisol) → coordinate reproduction & stress responses
  • Integral to myelin sheath → rapid nerve impulse conduction

Broad Importance of Lipids in Organisms

• Energy economy
  • Dense calorific value (≈ 38\,\text{J g}^{-1}) → efficient long-term fuel store
  • Light energy storage aids locomotive efficiency in animals / seed dispersal in plants
• Thermal insulation
  • Subcutaneous adipose layer; marine mammal blubber
• Mechanical protection
  • Cushioning around organs (e.g.
    kidneys)
• Structural & functional roles
  • Phospholipid bilayer formation; membrane fluidity modulation by cholesterol
  • Myelin sheath insulation → accelerates saltatory conduction
• Water balance
  • Metabolic water generation in desert fauna (e.g.
    kangaroo rat)
• Biochemical precursors
  • Vitamin D, bile acids, steroid hormones synthesised from cholesterol
• Waterproofing & defence
  • Waxy cuticle on plant epidermis limits transpiration & pathogen entry
• Solvent functions
  • Vehicle for absorption/storage of fat-soluble vitamins (A, D, E, K)

Ethical, Health & Real-World Implications

• Dietary saturated fats elevate LDL → cardiovascular disease risk; unsaturated fats may be cardioprotective
• Hydrogenation of unsaturated oils (trans fats) generates industrial products with negative health outcomes
• Sustainable plant oils provide alternatives to animal fats, impacting agriculture & environmental management

Concept Integration & Connections

• Carbohydrate (starch/maltose) breakdown parallels lipid catabolism: both rely on hydrolysis → smaller, metabolically labile monomers
• Amphipathic molecules (phospholipids, cholesterol) illustrate structure–function relationships central to membrane biology
• Energy storage strategies (glycogen vs triglycerides) reflect trade-offs between rapid mobilisation (carbs) and high-density storage (lipids)